Earth-boring tools for forming wellbores in subterranean earth formations may include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include a plurality of cutting elements that are fixedly attached to a bit body of the drill bit. Similarly, roller cone earth-boring rotary drill bits may include cones that are mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which it is mounted. A plurality of cutting elements may be mounted to each cone of the drill bit.
The cutting elements used in such earth-boring tools often include polycrystalline diamond compact (often referred to as “PDC”) cutting elements, also termed “cutters,” which are cutting elements that include a polycrystalline diamond (“PCD”) material, which may be characterized as a superhard material. Such polycrystalline diamond materials are formed by sintering and bonding together relatively small diamond grains or crystals, conventionally termed “grit,” under conditions of high temperature and high pressure in the presence of a catalyst (such as, for example, cobalt, iron, nickel, or alloys and mixtures thereof) to form a layer of polycrystalline diamond material, also called a diamond table. The diamond grit may comprise synthetic or natural diamond grains, or a combination of synthetic and natural diamond grains. These processes are often referred to as high temperature/high pressure (or “HTHP”) processes. The cutting element substrate may comprise a cermet material (i.e., a ceramic-metal composite material) such as, for example, cobalt-cemented tungsten carbide. In some instances, the polycrystalline diamond table may be foamed on the cutting element, for example, during the HTHP sintering process. In such instances, cobalt (or other catalyst material) in the cutting element substrate may be swept into the diamond grains or crystals during sintering and serve as a catalyst material for forming a diamond table from the diamond grains or crystals. Powdered catalyst material may also be mixed with the diamond grains or crystals prior to sintering the grains or crystals together in an HTHP process. In other methods, however, the polycrystalline diamond table may be formed separately from the cutting element substrate and subsequently attached thereto.
Upon formation of a diamond table using an HTHP process, catalyst material may remain in interstitial spaces between the grains or crystals of diamond in the resulting polycrystalline diamond table. The presence of the catalyst material in the diamond table may contribute to thermal damage in the diamond table when the cutting element is heated during use due to friction at the contact point between the cutting element and the formation. Polycrystalline diamond cutting elements in which the catalyst material remains in the diamond table are generally thermally stable up to a temperature of about seven hundred fifty degrees Celsius (750° C.), although internal stress within the polycrystalline diamond table may begin to develop at temperatures exceeding about three hundred fifty degrees Celsius (350° C.). This internal stress is at least partially due to differences in the rates of thermal expansion between the diamond table and the cutting element substrate to which it is bonded. This differential in thermal expansion rates may result in relatively large compressive and tensile stresses at the interface between the diamond table and the substrate, and may cause the diamond table to delaminate from the substrate. At temperatures of about seven hundred fifty degrees Celsius (750° C.) and above, stresses within the diamond table may increase significantly due to differences in the coefficients of thermal expansion of the diamond material and the catalyst material within the diamond table itself. For example, cobalt thermally expands significantly faster than diamond, which may cause cracks to form and propagate within the diamond table, eventually leading to deterioration of the diamond table and ineffectiveness of the cutting element.
Furthermore, at temperatures at or above about seven hundred fifty degrees Celsius (750° C.), some of the diamond crystals within the diamond table may react with the catalyst material causing the diamond crystals to undergo a chemical breakdown or conversion to another allotrope of carbon. For example, the diamond crystals may graphitize at the diamond crystal boundaries, which may substantially weaken the diamond table. Also, at extremely high temperatures, in addition to graphite, some of the diamond crystals may be converted to carbon monoxide and carbon dioxide.
To reduce the problems associated with different rates of thermal expansion in polycrystalline diamond cutting elements, so-called “thermally stable” polycrystalline diamond (TSD) tables have been developed. A thermally stable polycrystalline diamond table may be formed by leaching the catalyst material (e.g., cobalt) out from interstitial spaces between the diamond grains in the diamond table using, for example, an acid or combination of acids (e.g., aqua regia). In addition to leaching, some cutting elements may use particles of different average particle sizes to reduce the volume of interstitial spaces between grains of the diamond table. As the volume of interstitial spaces between the grains decreases due to the increased packing density of particles having different average grain sizes, a smaller volume of catalyst material may be present in the diamond table. All of the catalyst material may be removed from the diamond table, or only a portion may be removed. Thermally stable polycrystalline diamond tables in which substantially all catalyst material has been leached from the diamond table have been reported to be thermally stable up to temperatures of about twelve thousand degrees Celsius (1,200° C.). It has also been reported, however, that such fully leached diamond tables are relatively more brittle and vulnerable to shear, compressive, and tensile stresses than are non-leached diamond tables. In addition, it is difficult to secure a completely leached diamond table to a supporting substrate. In an effort to provide cutting elements having diamond tables that are more thermally stable relative to non-leached diamond tables, but that are also relatively less brittle and vulnerable to shear, compressive, and tensile stresses relative to fully leached diamond tables, cutting elements have been provided that include a diamond table in which only a portion of the catalyst material has been leached from the diamond table. For example, it is known to leach catalyst material from the cutting face, from the side of the diamond table, or both, to a desired depth within the diamond table, but without leaching all of the catalyst material out from the diamond table.